1. Field of the Invention
The present invention relates to a marine seismic source. In particular, it relates to a marine seismic source that consists of one or more airguns suspended from a float.
2. Description of the Related Art
A conventional marine seismic source of this type is disclosed in U.S. Pat. No. 4,686,660, and is shown schematically in FIG. 1.
The marine seismic source of FIG. 1 is intended to be towed behind a survey vessel. The source comprises a number of airguns 1 that are suspended from a float 2. The seismic source shown in FIG. 1 is intended to be towed to the left, and the left hand end 3 of the float is shaped so as to reduce water resistance when the source is towed in this direction. A tow line 4 to a towing vessel (not shown) is attached to the seismic source at the point A.
In the seismic source of FIG. 1 the airguns 1 are suspended from a supply harness 5. The airguns can be suspended singly or in a cluster of two or more airguns.
An airgun consists essentially of a container, normally a cylinder, that is charged with air at a high pressure such as several thousand p.s.i. In order to fire an airgun, a port in the airgun is opened by a solenoid-operated valve, thereby releasing the high-pressure air contained within the airgun. The released air forms a rapidly expanding bubble, so creating a powerful pressure wave. Information about the geological formations under the seabed can be deduced from the reflections of the pressure wave.
In the source of FIG. 1, the supply harness 5 consists of a plurality of electronic or electrical control boxes 7, each of which may be substantially as described in U.S. Pat. No. 4,757,482. The boxes 7 are linked by horizontal chains 8. One control box 7 is provided for each airgun or each cluster of airguns, and an airgun or cluster of airguns is suspended from its associated control box 7 by support chains 10 or other suitable support means. The control boxes 7 are linked by the horizontal chains 8, and the chains 8 bear at least part of the towing load when the source is under tow.
The supply harness is also provided with an air hose and electrical cables (not shown in FIG. 1). The air hose supplies air to the airguns from a source of compressed air on the towing vessel. The electrical cables supply electrical firing signals from an operator on the towing vessel to the solenoid-operated valve on each of the airguns, and return control and data signals from the airguns to the operator. The control boxes contain electronic or electrical circuits for processing the firing signals and actuating the solenoid-operated valve on the airgun or airgun cluster to fire the airgun(s), and also process control and data signals returned from the airgun(s) and forward them to the operator on the towing vessel. Thus, the supply harness 5 combines the two functions of, firstly, supplying the airguns with compressed air and electrical signals and, secondly, supporting the airguns and transmitting at least some of the towing load.
The harness 5 is suspended from the float 2 by chains 6 or other suitable supporting members. The towing point A is linked to the front control box by a further horizontal chain 8xe2x80x2, and is also supported by an inclined chain 9 from the front end of the float 3. In the array shown in FIG. 1, the chains 6 from the float 2 are attached to the control boxes 7, and the airgun or airgun cluster associated with each control box is suspended from the control box by further chains 10.
In alternative prior art marine seismic sources, individual control boxes are not provided for each airgun or cluster of airguns. Instead the control circuits are provided either in a single control box at the leading end of the harness, or on the towing vessel. In these sources, the harness 5 would consist of the horizontal chains 8, the air hose, and the electrical cables. The harness would be suspended from the float, with the support chains 6 attached directly to the horizontal chains. The airguns would be suspended from the harness.
Marine seismic sources of the general type shown in FIG. 1 have the disadvantage that the airguns 1 are mounted below the harness 5. Air is released violently from the airguns when they are fired, over a period of typically a few milliseconds. The released air initially forms expanding bubbles, and these rise to the sea surface. The volume of the bubbles oscillates as the bubble rises. Since the airguns are mounted below the harness, the air bubbles will impact the harness as they rise. As airguns become more powerful there is an increasing tendency for the violence of the expansion of the bubbles to damage the harness.
The present invention comprises a marine seismic source comprising: a float; an airgun; and a supply harness; wherein, in use, the airgun is suspended from the float such that its point of emission of air is at a shallower depth than the supply harness.
Since the airgun is suspended such that the point at which air bubbles are emitted is at a shallower depth than the supply harness, the impact on the harness of rising air bubbles produced when the airguns are fired is lessened. The risk of damage to the harness is therefore reduced.